A box social is a fundraising event. Various people bring boxes with food to the event, and the public bid on the different boxes. This may include a picnic with the maker of the food, or not. Very popular in the US around 1880-1920, apparently. Last heard in modern times when Marge in the Simpsons forbid Homer from having any festivites at Burns' house:
Homer: Marge, you're right. We do have to have a party.
Marge: Party? No! No parties!
Homer: What about... par-tay's?
Marge: No par-tays, no shindigs, no keggers, no hootenannies, no mixers, no raves, no box socials!

Box Social is also a punk band from the US.